1. Field of the Invention
The present invention relates to an image forming apparatus that uses a developing unit with a configuration that enables a developer fed along a predetermined circulation path to be carried on a moving surface of a developer carrying member to be used for development, and then returned from the surface of the developer carrying member to the circulation path.
2. Description of the Related Art
A developing device described in Japanese Patent Application Laid-open No. H9-160364 is known as one type of developing units related to the present invention. FIG. 1 depicts this developing device. As shown in FIG. 1, a developing device 900 includes a circulation path for circulatingly feeding a developer (not shown) that contains toner and magnetic carrier within a casing, a development roll 910, and the like. The circulation path includes a first developer containing chamber 901 and a second developer containing chamber 903, which are arranged side by side in a lateral direction. A developer contained in the first developer containing chamber 901, which is a part of the circulation path, is fed in the chamber space along a longitudinal direction by rotational driving of a first feed screw 902 in a direction of an arrow A in FIG. 1. The first developer containing chamber 901 and the second developer containing chamber 903 adjacent to each other are communicated at both ends thereof in the longitudinal direction. The developer fed to the end of the first developer containing chamber 901 in the direction of the arrow A in FIG. 1 with the rotational driving of the first feed screw 902 enters the second developer containing chamber 903 through a communication portion. The developer is fed in the second developer containing chamber 903 by rotational driving of a second feed screw 904 in a direction of an arrow B completely opposite to that of the arrow A. When the developer is fed to the end in the second developer containing chamber 903 in the direction of the arrow B, the developer enters most upstream of the first developer containing chamber 901 in the direction of the arrow A through a communication portion. In this way, the developer is circulatingly fed in the first and second developer containing chambers 901 and 903.
The development roll 910 is placed on a lateral side of the second developer containing chamber 903. The development roll 910 includes a development sleeve including a nonmagnetic pipe that is rotationally driven, and a magnet roller (not shown) that is unrotatably housed inside the development sleeve. The developer in the second developer containing chamber 903 is carried on the surface of the rotating development sleeve by a magnetic force produced by the magnet roller and fed to a development area in which the development sleeve and a photoconductor (not shown) face with each other. The surface of the sleeve is then developed with the developer, and the developer on the sleeve surface is returned to the second developer containing chamber 903. The toner concentration is reduced because the developer contributes to the development. A control unit (not shown) estimates an amount of toner consumed during the development of an image based on a result of calculation of the number of pixels in the image based on image information, and drives a toner supplying unit (not shown) for a time corresponding to a result of the estimation. In this way, supplemental toner is added to the toner in the first developer containing chamber 901 through a toner supply opening 915 located near an upstream end in a direction of developer feeding in the first developer containing chamber 901, thereby restoring the toner concentration of the developer. This toner supply enables to restore the toner concentration more promptly than in a configuration in which toner supply is performed after a toner concentration sensor detects a reduction in the toner concentration of a developer.
However, in a successive image-forming operation in which images are successively output onto plural sheets of recording paper, when image area ratios of the pages are greatly different from each other, it is difficult to stabilize the toner concentration of the developer for a reason explained below. That is, in the first developer containing chamber 901 that contains the developer being fed and not drawn up by the development roll 910, fluctuation in the toner concentration resulting from toner consumption associated with the development starts quite later than the consumption. When an image corresponding to one page is output for example, most of the developer used for the development of the image still remains in the second developer containing chamber 903 immediately after the output of the image. With subsequent rotation of the second feed screw 904, the development enters the first developer containing chamber 901 little by little, which gradually reduces the toner concentration of the developer in the first developer containing chamber 901. It takes a relatively long time that most of the developer used for the development in the development area has entered the first developer containing chamber 901. It means that the fluctuation in the toner concentration of the developer in the first developer containing chamber 901 due to the output of the image corresponding to one page continues for a long time. Nevertheless, an operation of supplying the toner corresponding to an amount of consumption for one page is performed for a short time. It implies that, immediately after a page with a high image area ratio is output, a large amount of toner is supplied to the first developer containing chamber 901 before the toner concentration of the developer therein is sufficiently reduced. This causes the toner concentration of the developer in the first developer containing chamber 901 to be higher than desired. When a page with a low image area ratio is then output, the developer with the toner concentration greatly reduced by the output of the previous page is drawn from the second developer containing chamber 903 to the first developer containing chamber 901 although only a small amount of toner is supplied to the first developer containing chamber 901. This causes the toner concentration of the developer in the first developer containing chamber 901 to be lower than desired. As a result, it becomes difficult to stabilize the toner concentration of the developer.